Presently, 2nd Generation (2G) systems and 3rd Generation (3G) systems are mainly being used. Currently, the standardization of Long Term Evolution (LTE) systems that are 4th Generation (4G) systems is in progress.
Because current LTE systems basically support packet service only, proposals for a more robust LTE standard include a Circuit Switch (CS) fallback service to provide a voice call to a user. The CS fallback service means a service for, when a voice call is generated for an MS that is camping on the LTE system, allowing the MS to handover to a 2G/3G legacy system and achieving a voice call connection.
Alternatively, a dual transceiver MS capable of simultaneously communicating with a Radio Access Technology (RAT) may receive packet service through the LTE system and simultaneously receive voice service through the 2G/3G legacy system. For example, when the MS is located in an area where the LTE system and a WCDMA system co-exist, the MS may simultaneously camp on the LTE system and the WCDMA system using a dual transceiver, and may monitor paging for the two systems at an idle mode as well as monitor paging of the WCDMA system even in course of receiving packet data service through the LTE system.
If an MS that supports the RAT gets out of a cell area of the LTE system by a user while simultaneously using data and voice services within the cell area of the LTE system, the MS should perform handover to a legacy system for a data call. At this time, for the data call, the MS should attempt handover to a legacy BS of voice call connection to perform successful handover. However, a serving BS that is in data call connection with the MS simply determines a handover target BS for the data call on the basis of a measurement report message received from the MS. Consequently, the serving BS may determine a different legacy BS, not the legacy BS of voice call connection, as a handover target BS. This leads to a failure of handover for the data call.